Of Wind Skirts and Winglets

I'm not sure when I noticed the first air skirt on a truck trailer; maybe three years ago. Now, it seems, you don't see many trailers that don't have these devices. In case you're not familiar with them, air or wind skirts are those angled blades mounted under the trailer that run most of the length of it from the tractor to the trailer wheels. They reduce drag by diverting the air from under the trailer, producing more laminar flow along the sides of the trailer.

Drag accounts for most of the energy required to move a vehicle and air skirts reduce drag considerably. One air skirt manufacturer, Freight Wing, claims a fuel economy savings of up to 7 percent. It doesn't sound like much, but it amounts to about 1000 gallons of fuel a year for a typical tractor trailer. At $3.50 per gallon, that ain't exactly chump change.

What's this got to do with airplanes? Maybe nothing, but maybe winglets—especially active winglets—may eventually become as ubiquitous on some airplanes as air skirts are on trucks. Both devices do the same thing: they save fuel by reducing drag. We ran a story earlier this week on a high school team that's proposed an adaptive winglet that they claim would adjust to various phases of flight to yield up to 10 percent in fuel savings. Their work is not even at the proof-of-concept phase yet, but one winglet design that's already out there is Tamarack Aerospace's active winglet. We've done a couple of videos on this and I flew it at the NBAA convention last month in Orlando.

Winglets do for airplanes what air skirts do for trucks, allowing, through reduced drag, the same speed on less fuel or higher speed on the same fuel. For airplanes, they can also increase climb rates measurably, which means that jets get to higher, more fuel-efficient altitudes sooner. As with a trucks, the fuel savings add up over time.

But that's where the truck/airplane analogy ends. It's one thing to bolt a fiberglass skirt under a trailer, quite another to add winglets to a wing with just enough structure to meet certification requirements, but not any excess to carry the additional bending loads imposed by adding a couple of feet to each wing. Tamarack's solution—a clever one—is to make the winglet active with a small, fast-acting flap that dumps the lift in flight phases where it would otherwise overload the wing structure.

What makes this possible, of course, is modern microprocessor technology that's sophisticated and reliable enough to calculate load factors on the fly and activate the winglet flap in fractions of a second. (In flight, this is quite noticeable. While ailerons move deliberately, the winglet flaps snaps to and fro instantaneously.)

Tamarack sees the principle market for its winglet as retrofits to jet aircraft which can benefit from the improved performance and fuel economy. In some airplanes, it only takes a nudge of a few percent to multiply range and performance considerations. Tamarack is making a kit for the Cirrus SR22, but they picked that airplane mainly as a test bed to prove the certification process. The system is too expensive to make much sense for small aircraft.

When you see such developments, it's natural to wonder why someone didn't think of this before. In the case of air skirts on trucks, I can only surmise that no one got around to the wind tunnel work until fuel prices reached a tipping point. After all, air diverters for the tractor roof have been around for decades. As for active winglets, the technology simply didn't exist to execute the idea. Now that it does, we'll see if it has legs.

Comments (21)

Uhm, I thought I had seen winglets on Boeing aircraft for quite some time now; as retrofits even. So what is the news?

The conformal winglets on the Boeings are fixed. Any loads they produce are transferred to the wing and they can not be optimized for all phases of flight. Fixed winglets are ubiquitous, but active winglets could become a common new control surface, increasing efficiency in all flight phases and usable on aircraft that cannot handle the loads of a fixed winglet.

The USAF tested fixed winglets on a KC-135 around 30 years ago at Edwards AFB. They determined that a 4-5% improvement in fuel economy would result from their use but then did not retrofit the KC-135 fleet because of up front costs. NOW, they're working on "green" fuels and still haven't installed the winglets on those airplanes. Just think of all the fuel the fleet could have saved during all those years!

Back in the 70's, around the time of the two fuel embargos, wind testing of large tractor trailer rigs was performed using gigantic fans on an abandoned original Camp Muroc runway. This resulted in the air dams you now see on almost all tractor cabs.

Pauyl, you need to clarify your statement on the trucks "They reduce drag by diverting the air from under the trailer, producing more laminar flow along the sides of the trailer." The air dams divert the moving air from under the trailer and reduce the parasitic drag around the axles, pipes, & rough edges. What's under the truck is a stagnant volume of air that moves with the truck but the high speed airflow remains outside the dam, which actually results in lower overall drag on the vehicle. The way it's worded is not quite clear.
Winglets are nice for high-speed airplanes in the Mach 0.8 region, but for us piston types they're just more parasitic drag and weight. On has to balance procurement/certification cost, weight, & added drag of the winglet to the net saving and compare it with something simple like just making the wing longer to increase aspect ratio and comparing the difference. Sometimes the higher AR is just as much of an improvement as the winglet, but then you need the active ailerons to unload the wing in turbulence lest you break the wing off. That means you now have to certify the electronics and mechanism, build it, install it, and maintain it. For the airline, it is probably something worth considering. For us? Nah.

Does anyone have a good link to how the winglets work or what forces are present on them? I would think they would provide a largely lateral (along the spar of the wing) force, with a slight component in the upward direction (against gravity in cruise flight). But I've never seen an explanation and am curious what makes them tick.

On older and slower planes, we used to put flat plates at the tip, or 'drooping' wing tips, or whatever the flavor of the day was, even on a Cub.

I flew two otherwise identical J-3s, one with and one without the 'drooped' tips for STC requirements. If there was a difference, it was not measurable. But we got the STC anyway.

I also thought it interesting that the Cessna 310 'tuna' tanks (think flat end-plate) were just as efficient as the later canted tanks. Even Cessna admitted that the change was marketing driven for more modern styling.

When making a Tripacer STOL there was a vertical fence across and a flat plat at the end of each wing. The owners also wanted to install a slat to the leading edge in a bid to better STOL capabilities. These were farmers with short runways in the bush so anything to make the takeoff and landings better was looked at. I had a V35 with tip tanks that was supposed to reduce wing tip vortices along with other advantages. If it did I don't know didn't fly the aircraft before the tanks were installed.

So why shouldn't the winglets work? Would sure like to know if they do.

You missed the salient point about SPEED. Drag reduction at 55/60mph is not a pressing issue. Now that open road speed limits are 80 mph, now it's enough of an issue to address trailer drag. Aero has not changed, speed limits have.

All the wings and skirts and fins don't have much effect at slower speeds. I can't fly 55.

This is incorrect. Only a portion of the U.S. has limits that high, specifically Utah and portions of Texas. The vast majority of the populated U.S. is at 70 MPH. The midwest has some 75 MPH and has had for quite some time. The northeast is mostly 65 MPH.

The point you miss is fuel costs, which have gone up much more than the incremental Delta in speed limits. Even at 55 or 60, the lower drag saves fuel. Otherwise, operators wouldn't have bothered with the front air dams, which virtually all trucks have.

Fuel has REMARKABLY stayed relatively stable over the years if you adjust for inflation, yet engine efficiency has risen.
Since it's a business decision, I wonder if OTHER higher cost of business are driving the adoption? Personnel and insurance costs have skyrocketed. Maybe aero is just a way to cut costs to try to "stay even" with the things that are raising the real cost of doing business?

Five years ago, on-road diesel averaged $2.88. Now it's $4.15. That's higher than the rate of inflation by about a buck. Diesel truck economy gains continue. They average between 7 and 8 MPG for state-of-the art engines.

The difference between 70 and 80 MPH on fuel economy is about 14 percent. You might get back half of that with aerodynamic improvements, if you look at the data from the companies that sell these things.

So in 2007, if you saved 1000 gallons a year, you saved about $2800. Now, you save $4200--a 50 percent Delta. Somewhere in there, the tipping point occurred and air skirts became common.

What's odd is that high cab mounted (and front trailer mounted) aero aids seem to have all but diapered. That should be where most of the drag can be reduced. The skirts (Low on the trailer behind the tractor) is probably only helps in heavier crosswind conditions where the rear wheels are out of the slipstream...

WInglets aren't just for fast planes. If they were, you wouldn't see them on gliders.

Just like any other lift-generating surface, a winglet has an angle of attack. For several reasons (efficiency, structural requirements), some angles of attack are better for certain airspeeds than others; regrettably, a winglet's AoA is fixed (or more accurately: it can't be controlled directly). A few aircraft manufacturers omit winglets for this very reason (notably in gliders: Windward Performance).

The Tamarack solution to this conundrum is to "cancel out" the winglet via changing the AoA of the adjacent bit of wing. Fair enough---it's probably hard to pitch the winglet itself or fit a moving control surface inside.

In any case, it's important to realize that even if structural integrity is not an issue, a fixed winglet is a lift generating device tailored to a particular airspeed range selected by a designer, and that can include low speeds and high speeds (but not both at once). It may look cool all of the time, but it only helps some of the time.

The last big fuel inflation in the early '70s initiated the aero skirts and fairings on trucks. AeroEnvironment made their first profits on these truck structures. In 1968, the Wichita State had a semi model in the wind tunnel where our aero class performed analysis. It makes even more sense to apply aerodynamics to vehicles today.

Um, does no one remember the TriStar? When they wanted to increase the wingspan but didn't want to increase the wing structure?

Didn't the long-wing version of the TriStar get certified with active ailerons that would dump the load when necessary?

I'm pretty sure I'm not remembering some long-ago dream. I definitely remember a long-wingspan modification, or version, of the TriStar.

This was before computers became so ubiquitous and cheap, but the TriStar, again as I recall, was the first U.S. civilian airplane to be certified for CAT III approaches, so it already had a lot of computer capability for its time.

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Picture of the Week

As aviation photos go, this was the best this week but there are some great beauty shots when you click through. In the meantime, congratulations to Daniel Gillette for this very nice photo he calls Sunset Pitch-Out. The photo is copyrighted by Gillette.